1. Technical Field
This disclosure relates to methods for promoting neurite outgrowth after nerve cell loss as a result of central nervous system (“CNS”) injury or disease. In particular, chondroitinase AC and chondroitinase B are used to promote neurite outgrowth.
2. Description of Related Art
After a spinal cord injury in the adult mammalian central nervous system (CNS), the inability of axons to regenerate may lead to permanent paralysis. An injury-caused lesion will develop glial scarring, which contains extracellular matrix molecules including chondroitin sulfate proteoglycans (CSPGs). CSPGs inhibit nerve tissue growth in vitro, and nerve tissue regeneration at CSPGs rich regions in vivo.
A number of molecules, and specified regions thereof, have been implicated in the ability to support the sprouting of neurites from a neuronal cell, a process also referred to as neurite outgrowth. The term neurite refers to both axon and dendrite structures. This process of spouting neurites is essential in neural development and regeneration, especially after physical injury or disease has damaged neuronal cells. Neurites elongate profusely during development both in the central and peripheral nervous systems of all animal species. This phenomenon pertains to both axons and dendrites. However, adult neurite regrowth in the CNS is increasingly lost with evolutionary progress.
Various polypeptides, especially cell adhesion molecules (CAMs), have been known to promote neural cell growth. While early efforts in this area of research concentrated on the adhesion-promoting extracellular matrix protein fibronectin (FN), other polypeptides have also been found to promote neural growth. For example, U.S. Pat. No. 5,792,743, discloses novel polypeptides and methods for promoting neural growth in the central nervous system of a mammal by administering a soluble neural CAM, a fragment thereof, or a Fc-fusion product thereof. U.S. Pat. No. 6,313,265 discloses synthetic polypeptides containing the pharmacologically active regions of CAMs that can be used in promoting nerve regeneration and repair in both peripheral nerve injuries as well as lesions in the central nervous system.
While helpful, the use of regenerative proteins alone may not be sufficient to effect repair of a damaged nervous system.
One area that has been determined to be of significance in the repair and regeneration of cellular tissue, including neural tissue, is the extracellular matrix. Extracellular matrix proteins (“EMPs”) are found in spaces around or near cells of multicellular organisms and are typically fibrous proteins of two functional types: mainly structural, e.g., collagen and elastin, and mainly adhesive, e.g., fibronectin and laminin.
During approximately the past two decades, the base knowledge of cell adhesion and migration in extracellular matrices (ECMs) at the molecular level has expanded rapidly. The action of enzymes and other polypeptides which degrade components of the extracellular matrix and basement membranes may facilitate the events of neural repair by a variety of mechanisms including the release of bound cytokines and by increasing the permeability of the matrix, thereby enhancing the mobility of mediator molecules, growth factors and chemotactic agents, as well as the cells involved in the healing process. For example, U.S. Pat. No. 5,997,863 discloses the use of glycosaminoglycans to manipulate cell proliferation and promote wound healing.
ECM molecules include the inhibitory CSPGs. Components of the CSPGs have been identified as the glycosaminoglycans, chondroitin sulfate (CS) and dermatan sulfate (DS). Removal of these inhibitory molecules would allow neurites to regenerate and reinnervate an area after physical injury or disease.
Previous studies have found that chondroitinases can lyase and degrade CSPGs and, including, CS and DS. One study found that chondroitinase ABC removed glycosaminoglycan (GAG) chains in and around lesioned areas of rat CNS, in vivo. The degradation of GAGs promoted expression of a growth-associated protein, GAP-43, indicating increased regenerative propensity in treated cells. However, this growth-associated protein is associated with regeneration in peripheral, but not central, nerve injuries. Applications of chondroitinase ABC to an injured corticospinal tract (CST) prevented axon retraction from the affected area and promoted more axon fiber growth than the control, with some axons arborizing into gray matter. Regenerated CST axons established functional connections. (Bradbury et al., Chondroitinase ABC promotes functional recovery after spinal cord injury, Nature 416: 636-640 (2002)). Another study found that in vitro chondroitinase ABC treatment of rat spinal cord regenerated neurons on a tissue section substrata. This study observed that degradation of CSPGs may promote the neuro-stimulatory effects of laminin. (Zuo et al. Degradation of chondroitin sulfate proteoglycan enhances the neurite-promoting potential of spinal cord tissue, Exp. Neurol. 154(2): 654-62 (1998)). In a later study by same primary researcher, it was reported that injection of chondroitinase ABC at the site of nerve damage degraded CSPGs and increased the ingress of axonal sprouts into the basal laminae of the distal nerve segment, which may be by enabling more latitude in growth at the interface of coapted nerve. (Zuo et al. Regeneration of axons after nerve transaction repair is enhanced by degradation of chondroitin sulfate proteoglycan. Exp. Neurol. 176(1): 221-8 (2002)). The same group of researchers also found chondroitinase ABC treatments regenerated axons on into acellular grafts at a much higher rate than the control grafts. (Krekoski et al., Axonal regeneration into acellular nerve grafts is enhanced by degradation of chondroitin sulfate proteoglycan. J. Neurosci. 15:21(16): 6206-13 (2001)).
The use of chondroitinase AC and chondroitinase B would be advantageous to promote neurite growth in mammals because these chondroitinases strongly promote neurite outgrowths directly in the CNS, itself, as well as in the peripheral nervous system.